1 Introduction

Inter-trochanteric femur fractures are a common injury seen in elderly, the understanding of which is necessary for devising treatment options which will decrease the risk of mortality and morbidity arising out of this injury.¹ Currently out of 280,000 fractures occurring annually nearly half of these fractures are due to inter trochanteric fractures.² The incidence of hip fractures is increasing in many countries,³ and the total number of hip fractures is estimated to be more than 5 million by 2050.⁴

Intra-medullary device, one of the modes of fixation of these fractures, has shown a significant increase in its use (from 3% to 67%) over the last 2 decades in the United States and Europe.⁵ Native femoral bowing poses a big challenge to surgeons for adequate fit of such intra medullary devices. Racial and ethnic group differences alter the femur morphometry further. The osteometry of Asian skeleton differs considerably from that of western population; often requiring implant modifications for better fit of the intra medullary implants.⁶ These intra-medullary nails for proximal femur fractures are usually available in both long and short sizes. Shorter nails provides shorter surgical time, less blood loss and lower transfusion rates as compared to the longer nails and often preferred by surgeons.⁷ However, these short proximal femur nails, unlike the long ones, are usually straight with no curvature in antero-posterior plane and always leave a chance of the nail tip impinging against the anterior cortex of femur.^8–12

The history of these proximal femur nails (PFN) dates to 2004, when it was introduced by Arbeitsgemeinschraft fur Osteosynthesefragen/Association for the Study of Internal Fixation (AO/ASIF) group as PFNA(proximal femur nail anti rotation).¹³ Higher incidence of mismatch was seen with these PFNA in Asian/Caucasian population, and later in 2008 a modified version of PFN was introduced as PFNA II (Asian version) with a single head neck helical blade.¹³ However, similar complications of anterior thigh pain and anterior cortical encroachment persisted with this nail modification as well.¹⁴ Even within the same ethnicity, femoral bowing showed a strong correlation with age, height and BMD (Bone Mineral Density) varying from subject to subject.¹⁵ Short PFN usually comes in the following sizes – extra-small (170 mm/180 mm), small (200 mm/220 mm) and standard (240 mm/250 mm), sizes varying according to the manufacturers, with all these nails being straight in sagittal plane.¹³

We hypothesized that non anatomical, straight short proximal femur nails have a tendency to impinge on the anterior cortex of the femur and can cause anterior thigh pain in patients with increased femoral bow. We further hypothesize that nail length and distal nail tip diameter plays a role in this anterior cortex impingement of femur by the nail tip. The aim of the study is to determine the degree of mismatch, determine the extent of anterior encroachment by the nail tip and the possible measures to avoid them in Indian population.

2 Material and methods

A retrospective and prospective study was designed to evaluate the functional and radiological outcomes after proximal femur nailing with short nails and to assess the degree of mismatch with the anterior bow of femur. The study was carried out from January 2019 to April 2021, after obtaining institutional ethical committee approval and appropriate consents from all participating individuals in the study.

Patients were assessed clinically and radiologically using X-ray Hip Antero-posterior (AP) view preoperatively. AO/OTA (Orthopaedic Trauma Association) classification was used to grade the fractures.¹⁶ Only 31A2 and 31A3 fractures involving patients in the age group of 40–80 years were included in the study. Any high energy trauma, pathological fractures, open injury, patients with degenerative arthritis of hip, non ambulatory status prior to injury, and patients lost to follow-up were excluded from the study. A total of 80 patients were finally included in the study.

All fractures were fixed on a fracture table (closed/open reduction) and fixed with short PFN ranging from 170 to 250 mm including both Indian and Western implants. Appropriate distal nail diameters were used as per the operating surgeon choice ranging from 9 mm to 12 mm. Post-operative X-rays (Hip AP view/Lateral views) were taken on post-operative day 2, at 1 month and at 6 months follow-up. Reduction quality was assessed at 6 months on X-rays using the Tip Apex Distance (TAD) and Neck Shaft Angle (NSA), digitally using PACS (picture archiving and communication system) tools.^17,18 Follow-up patients with acceptable radiological reduction parameters (TAD <25 mm) were included in the study.

Post-operatively, all patients were started on in-bed mobilization. Pelvic bridging, active knee and hip flexion along with active ankle pumps formed the basic rehabilitation protocol for the initial two weeks. Patients were encouraged for mobilization with walker support from post operative day 3. Prolonged recumbency was avoided and appropriate deep vein thrombosis prophylaxis was advocated for all frail patients. Post suture removal, patients were encouraged full weight bearing mobilization independently. On follow up at one month, it was ensured that patients were ambulating with no complaints of pain.

Radiological analysis was done at 6 months post-operative hip lateral view X-rays, digitally using PACS tool by the principal investigator himself. Two important parameters were evaluated- Angle at Distant Axes (ADA) and Nail Tip Position (NTP). The angle between the longitudinal axis of the nail and central anatomic axis of femur at the distal nail tip will be measured digitally on lateral radiograph. This angle will be defined as Angle at Distant Axis (Fig. 1). Patients will be defined into 2 groups according to the ADA-Group 1 with ADA <4°, Group 2 with ADA> 4°. Magnification factor was calculated by measuring the nail length on true lateral view x-rays. Original lengths of the nails used in the patients were documented from patient's discharge summaries/implant details. Dividing it by the original nail length gave the magnification factor, to be adjusted in all measurements done digitally. Centre point at the distal nail tip was marked. A point was taken 3 cm above and below the nail tip, along the axis of the nail (after magnification correction). Perpendiculars from the anterior and posterior cortices were drawn through these points. Midpoint of these 2 perpendiculars were pointed and line joining these 2 points gave us the anatomical axis of femur at distal nail tip angle between the mechanical axis of nail and anatomical axis of femur at nail tip was then drawn, which was taken as Angle of distant axis. Nail tip position (NTP) was graded on lateral view hip X-ray using the scale as mentioned in Table 1.¹³ Following radiological analysis, functional analysis of the patients was done using modified Harris Hip Scoring (HHS) system.¹⁹

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Fig. 1 Hip lateral view X-rays depicting the method to calculate ADA.

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Statistical analysis was done using SPSS software (IBM version 21). Descriptive statistics and frequency tables were analyzed for patient demographics, implant characteristics, radiological and functional outcomes. Chi-square test was used to check the significance of ADA with thigh pain, as well evaluated for co-relation of thigh pain with NTP, nail diameter and nail size. Significance was considered at p values < 0.05.

3 Results

3.2

3.2 Analysis of implant morphometry

Proximal femur nails with lag screws were used in 53 (66.3%) patients while helical blade was used in 27 (33.7%) patients. Nail length of 200 mm was the most widely used nail in the study (40%), followed by the 180 mm nail which was used in 26% cases. Mean nail size was 201.87 mm. Distal nail diameters of 10 mm and 9 mm were used to stabilize the fractures in 49 (61.2%) and 25 (31.2%) patients respectively (Fig. 2), with a mean nail diameter of 9.76 mm.

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Fig. 2 Diagrammatic representation of distribution of nail sizes and nail diameter depicting (a) 200mm nail length and (b) 10 mm nail diameter being the most commonly used nails in the study.

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4 Discussion

Our study evaluated 80 patients over a period of 2 years with inter trochanteric fractures who were treated by the short straight PFNs. The study evaluated whether the straight nails impinge on the anterior cortex of femur due to the anatomical bow of femur and secondarily also evaluated the clinical relevance of this impingement. All the complications like – anterior cortical abutment, encroachment, erosion and penetration have been well reported previously in literature and are related to mismatch of radii of curvature between the femur and nail.¹³ In treatment of proximal femur fractures, anterior cortical encroachment of the cephalo-medullary nails have been described before,^9,11,12 however, the relation between the short PFN and anterior cortical impingement is not documented well in literature. In our study we tried to assess the degree of mismatch of these short PFN through radiological and functional analysis.

The mean ADA was found as 4.19 ± 1.45, and was more than 4° in 37 patients (46.3%) in our study. A similar study conducted by Yildiz et al., in 2018 on 111 patients yielded an ADA of 4.9 ± 1.5; with 64 patients having ADA more than 4°.⁸ ADA was considered as one of the parameters to assess the nail tip mismatch in femoral canal. This mismatch was clinically interpreted as patients coming back with incidence of thigh pain on follow ups. In our study we compared higher degrees of ADA with incidence of anterior thigh pain (Table 3). Out of the 20 patients with anterior thigh pain, 18 patients had ADA>4° (p < 0.001).

The distal nail tip was also graded using NTP grading system in our study. A total of 72 cases were seen with nail tip anteriorly located in the canal, with NTP grade 1 and above. Grade 2 and above included 49 cases where the nail tip abutted against the anterior cortex of femur. A study of 158 cases done by Chang et al., in 2014 had 118 cases with anterior location of nail tip in canal and 55 cases abutting against the anterior cortex.¹³ In our study, 25 cases had NTP grade 3 and above. While correlating the degree of NTP and the incidence of anterior thigh pain (Table 4), we had 16 patients with thigh pain and found to be statistically significant (p < 0.001); depicting higher grade of NTP is associated with increased incidence of anterior thigh pain.

Our study included both PFNA and PFNA II short nails (both Indian and Western implants) ranging over 170–250 mm. PFNA was first introduced in 2004 and has been a quite popular method of fixation of trochanteric fractures. However, these implants were designed keeping in mind the proximal femur geometry of the western population. Siwach R in his study on proximal femur geometry highlighted how the Asian, rather the Indian femoral geometry, is different from the western population-having shorter femoral neck, smaller femoral neck angles, shorter lengths of femur and increased bowing of the shaft.²⁰ Later on AO group introduced a modified Asian version of PFNA with revised sizes and geometry.¹³ PFNA II came to market with narrowed proximal diameter of 16.5 mm, reduced valgus angle to 5° and a flattened lateral surface. These modifications did answer to many complications with the previous PFNA where valgus impingement of lateral cortex and femoral fractures were common in Asian population, but remained straight without any compensation for the normal anterior bow of femur.^21,22 However, in our study the differences in functional outcomes between these two group of implant were not significant.

Egol et al. in his study of proximal femur nails and mismatch with anterior bow of femur mentioned about varying degree of bowing according to age, osteoporosity, short stature and ethnicity of the individuals.²³ The authors also mentioned that the problem while negotiating these intra-medullary devices are more in cases of per/inter-trochanteric fractures(AO/OTA 31A) as compared to subtrochanteric or diaphyseal or distal end fractures, in which the mismatch in curvature of the nail and femur can be accommodated by sagittal angulation at the fracture site or by over reaming.¹³ The short cephalo medullary nails are designed in such a way that the distal tip should not cross the apex of the proximal femur.¹³ The bowing of the femur is considered to start near the isthmus, almost at the junction of proximal and middle one third of shaft of femur. So avoiding straight nails, traversing beyond this point will help to keep the fear of this mismatch at bay. Short stature, obese individuals with advanced age have a tendency of aggravated bowing of the femur, where the bowing may start even before the isthmus junction. In such individuals proper pre-operative planning by taking a full length femur X-rays will help, wherein we can fix the fractures using a long PFNs having a native bowing to prevent any adverse mismatch that is likely with the straight short PFNs.

The short PFNs ranging from 170 mm to 250 mm were included in our study. The mean nail size was 201.87 mm ± 23.06. We correlated the nail tip anterior location and nail tip abutment with the increasing sizes of the PFN nails (Table 5). It showed the anterior cortex abutment rose from 42% with 180 mm nails to 80% with 220 mm nails, thus playing a significant part in mismatch and leading to anterior thigh pain in patients. 53 patients had a nail length of 200 mm and above, with 16 out of the 20 anterior thigh pain patients belonging to this group (p < 0.01). Chang et al. in his study on short PFNA II (Synthes) and mismatch with bow of femur included 38 cases with 170 mm nail, 115 cases with 200 mm nail and 5 cases with 240 mm nail length.¹³ An increase in nail tip anterior abutment was seen to increase from 46% in 200 mm nail length to 80% in 240 mm nails. The study also showed an increasing anterior location of nail tip with increasing nail size, with 71.4% in 170 mm nail to 80% in 240 mm nails. All these nails are available in distal nail diameters ranging from 9 mm to 12 mm. In our study, we tried to correlate the relationship between the distal nail diameter and incidence of thigh pain as well. We had a mean nail diameter of 9.76 mm ± .57. We had 55 patients with nail diameter above 10 mm, and 16 patients out of the 20 anterior thigh pain patients were in this group (p < 0.01). We found a statistically significant relationship with increasing distal nail diameter and incidence of thigh pain in our study group (Figs. 3 and 4).

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Fig. 3 Case example showing (a) right hip inter-trochanteric fracture in a 62 year old male who was operated by 240 mm PFNA and nail diameter 10 mm. Immediate postoperative X-rays taken on day 2 as shown in (b) and (c). Radiological analysis was done at 6 months on AP and Lateral hip X-rays which showed a TAD of 17.5 mm and NSA 129° as in (d) and (e). ADA measured 6.12° with NTP grade 3 (f) and HHS of 66, with patient complaining of anterior thigh pain on mobilizing.

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Fig. 4 (a) Post-operative X-rays of 68 year male, at 6 months follow-up - AP view showing a 240 mm PFNA2 implant with distal nail diameter of 10 mm (b) Lateral X-rays showing ADA of 6.16° with NTP grade 3 on radiological analysis. (c) Another 62 year male with a trochanteric fracture was operated with 240 mm PFNA2 with a nail diameter of 10 mm - AP view and (d) Lateral view X-rays on 6 months follow-up (e) Radiological analysis showed ADA 3.95° with NTP grade 2.

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Treatment of trochanteric fractures is not without complications. The most frequent complications includes-varus collapse of proximal femur, cut out of lag screw/helical blade, shortening of femur, screw fractures, peri implant fractures, secondary fractures of the greater trochanter, Z effect or reverse Z effect with 2 lag screws. Our study had 3 cases with a femoral shortening in the range of 1–2 cms, while 5 patients had shortening around 1 cm.We had 2 cases of cut out with lag screw and helical blade each, and both had negative antero-medial cortical support on post reduction/fixation X-rays, resulting in varus collapse at fracture site. Qin Shao et al. highlighted how negative support in unstable trochanteric fractures have a higher chance of failure than a positive or a neutral reduction.²⁴ However, on radiological analysis of both these cut throughs, ADA was found to be less than 4° with NTP grade 1. We had 45 patients with neutral, 25 patients with positive and 10 patients with negative reduction in our study. We also had one case with a peri-implant fracture at level of distal screw site. Radiological analysis of this patient revealed an ADA of 4° with NTP grade 2.

We acknowledge certain limitation in our study. We had only 80 patients in our study which might seem just inadequate in comparison to the scantily reported studies. Secondly, all the radiographic analysis was done digitally by the principal investigator himself and hence adequate blinding was not done. However, all the measurements being computer generated using PACS tools, there is not enough scope for inter-observer bias. Lastly, we did not take into account the age, height and BMD of the patients; all the factors that play a significant role in anterior bow of femur.

5 Conclusion

There is a mismatch of short straight proximal femur nails with anterior bow of femur in Indian population. Use of a short nail (170 mm/180 mm) and a small distal nail diameter (9 mm) will avoid this mismatch. Full length radiographs of the femur in short stature individuals and proper pre-operative planning may help to avoid its incidence further and hence the complications arising out of it.

Authors’ statement

All authors have read and approve of the final submitted version of the manuscript.

Ethical review committee statement

Institutional ethical committee approval was taken for this study.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.

Declaration of competing interest

The authors declare they have no conflict of interest.

CRediT authorship contribution statement

Kaustav Mukherjee: Writing – original draft, Data curation. Tarun Prashanth K.R: Conceptualization, Methodology, Software. Thirunthaiyan M. R: Visualization, Investigation, Supervision. Dorai Kumar R: Writing – review & editing.